A semiconductor device is formed by mounting a semiconductor chip on a circuit wiring board (lead frame, board, tape, etc.) and connecting electrodes on the semiconductor chip and electrodes on the circuit wiring board by bonding wire for semiconductor device use (hereinafter, also simply referred to as “bonding wire”).
As bonding wire for semiconductor device use, fine metal wire with a wire diameter of 20 to 50 μm or so is being used. One end of a bonding wire is formed into a ball by heating the tip of the wire to melt, then this ball part is press bonded on an electrode of the semiconductor chip by pressure bonding with ultrasonic and heat. The other end of the wire is bonded on an electrode of the circuit wiring board by pressure bonding with ultrasonic.
As the material for bonding wire, in the past, high purity Au (gold) or Au alloy has been used. However, Au is expensive, so another type of metal with a cheaper material cost is desired. As a low cost wire material other than Au, Cu (copper) is being studied. Compared with Au, Cu is more easily oxidized, so PLT 1 describes the example of a two-layer bonding wire comprised of a core material and a covering layer (outer layer) where Cu is used for the core material and Pd (palladium) is used for the covering layer. Further, PLT 2 discloses bonding wire which has a core material made of Cu or Cu alloy, a covering layer having Pd as its main ingredient at the surface of the core material, and an alloy layer which contains Au and Pd at the surface of the covering layer.
A Cu wire or Pd-coated Cu wire is high in hardness after bonding, so a material with a lower hardness is being demanded. As an element which has electrical conductivity equal to or better than Au and which is lower in hardness than Cu and, furthermore, which has oxidation resistance, Ag (silver) may be mentioned.
PLT 3 discloses an Ag—Au—Pd ternary alloy-based bonding wire mainly comprised of Ag. The bonding wire is heat treated for annealing before continuous die drawing, is drawn continuously by die drawing, then is heat treated for tempering, and is bonded by ball bonding in a nitrogen atmosphere. Due to this, even if used in a harsh environment of a high temperature, high humidity, or high pressure, it is considered possible to maintain bond reliability with aluminum pads.
PLT 4 discloses an Ag—Au, Ag—Pd, and Ag—Au—Pd alloy wire material comprised mainly of Ag. The center part of the alloy wire material contains elongated crystal grains or isometric crystal grains. The other parts of the alloy wire material are comprised of isometric crystal grains, so the crystal grains, including annealing twins, become 20% or more or the total. The objective is to improve the quality and reliability of the package products.
The distances between adjoining bonding wires have been made narrower, that is, the pitches have been made increasingly narrower. As demands on bonding wires corresponding to this, increased fineness, increased strength, loop control, improvement of bondability, etc. are being sought. Due to the higher density of mounting of semiconductors, the loop shapes are becoming increasingly complicated. Loop shapes are classified by loop height and bonding wire length (span) as parameters. In the latest semiconductors, in increasing cases, single packages contain high loops and low loops, short spans and long spans, and other opposite types of loops mixed together. Realizing this by a single type of bonding wire requires strict design of the material of the bonding wire.
As the properties of wire which is used in mass production, satisfying general properties like stability of loop control in the bonding step, improved bondability, suppressed wire deformation in the resin sealing step, and long term reliability of the bonded parts, for instance, has been desired so as to enable cutting edge demands for narrow pitch connection, multilayer chip connection, and other high density packaging technology.